Solutions and methods for forming protective coatings on titanium



United States Patent Office 3,041,215 Patented June 26, 1962 ign NoDrawing. I'led Feb. 7, 1956, Ser. No. 563,865

priority, application Great Britain Feb. 7, 1955 1 Ch'lns. (CL 14S-6.14)

This invention relates to the formation of adherent protective coatingson titanium and more particularly to chemically formed fluoride coatingson titanium and titaninmbasealloys.

It is an important object of the invention to provide improved solutionsand methods for chemically forming strongly adherent coatings onsurfaces of titanium and titaniuxnbasealloys.

Otherimportant objects oftheinventionaretoprovidearclesoftitaniumandoftitaninmalloyswhichhaveiormedthexeoncharacteristiccorrosionresistantcoatingsandtoprovidesaidarticlesinsuchcoatcdformwherebydieymaybedeformedandooldworkodwithoutiniuryto tbesubsuate.

'lheaboveandotberobectsoftheinventionwillbecome apparent during thecourse of the following description.

Inacoordancewiththenventimtitanirnnandtitaniumbasealloyscanbemnvidedwithauniformadherem uoridecoating whichfacilitates drawingandoold workingofmemetalandwhichimpartsdaeretosuperiorcorrosionresstance.Forrnaonofthecoatingoccursfrom asurfaeeofthemetalwithanaqueousacidicsolntioncontainingtbellnorideionandionsofoneor moreofthealkalimetals.Forpurposesofthepresent inventiomammonumisinclndedinthetermalkalimetals.ItisessentialthmapHbetweenaboutZandbemaintainedforthesolutionandpreferablythepliismaintaincdbetweenabout25and5.

Ithasnowheenfonndthatacertaingroupofcomponndshaventhennusualelectofreactingwithttanirnnandtitaninmbealloysinthepresenceoftheandtheiomofthealkalimetalstoforman ysnperorandfromwhatappeaatypeofoftheooatingremainsunknowmbutitisknownfromitspropertiesandeectstoheyadherentandproteotiveandtobeofsolntions havingthementionedandoneoftheofthis eertaingroup.Thedistimadvamagesof-theinvention arerealzedarlyunderthe i thatthepartandpreferablyallotftheaa'dityofthesohrtionis thrived fran one ofthis certain group ofoompounds.

Ammtheinvtiomth'certaingmupof arealladdsandallexhibttoamarkeddegmeIethedescribedbeneficialpropertywhcnllstdilllhYPetheprinpalsouroeofaddity.'Diegmpofaddssofomtoexhibtthispropertycomprisesformhydrotluocacidwhchhasitsowndistinctwinappeanandarsohydxhkacacd.

andsimilarlytheirehassepamteadvantages.Discoveredtobeequivalanoftheinm'-gancaddsintheireectonhmalsohavingseparaareorganicaa'dswhichhaveaprimaryumstantls than 3x10" butgreater than 1x10- Bygmaryconstantismeantthe sat approximately mom "wmlhmthereare oneatnms. Ofthelattermoreadds,i.e.,theorganicacidsewhichareuserlfnrforming coatings of thepresent invention, the following are preferred: succinic acid; rnaleicacid; salicylic acid; tartaric acid; phthalic acid; acetic acid; benzoicacid; and citric acid.

Acording to a feature of the invention, advantages are realized whensolid materials are used as the source of one or more of the above groupof acids and/or the other ingredients of the solutions. The solidcompositions are much less corrosive than the solutions so thatpackaging and handling are facilitated, and economies in storage andtransportation are effected.

The coatings of the invention are formed by contacting the surface oftitanium and the like with an aqueous solution containing one or more ofthe adds of the above group, viz., hydrouonc acid. and hydrochloricacid, nitric acid or sulphur-ic acid and an organic acid having aprimary dissociation constant less than 3 X 10-2 but greater than lxlO"at a pH between 2 and 6; the solution also containing at least 0.5%iluoride ions and at least 0.5% sodium or potassium ions or both.Preferably the concentration of the above group of acids is maintainedso that the pH of the solution is vnthin the optimum range of 2.5 to 5.The anion radical of these acids which disociates imm the hydrogenradical appears to be the most important factor accounting for thesimilarity in elect on coating of the acids when present in thesolution, and consequently while including other acids may be present inminor amounts, it is imperative that the greater part, and preferablyall of the acidity of the solution is derived from one or more of theacids of the above group.

It has been found that the coatings obtained with solutions containingsodium are formed more rapidly than those obtained with solutionscontaining potassium. If ammonium or lithium ions or both are used inplace of sodium or potassium, their concentration must besubstantiallyincreased. and,intbe caseofammonium,thecon centraton oftheammonium ion should be at least 2%.InthecaseoflithiumitispreferabletonseatleastZi but it is possible to useless. Unless otherwise' indicated. percentagesasgiventhroughoutthearebyweight percent. Mixtures of the above alkali maybeusedandinthissetheminimnmconcentratlonwil'llre somewhere between theminimum given above for the iom separately dependrng on the relativeproportions of each in the mixture. Excellent coatings have been formedfrom concentrations of 8% alkali metal `non.

Thc Solution preferably should be y free from calcium andotherionsknowntoformnondesthat are insoluble in aqueous solutions' as othervme'theuorideionoccursby allowingtlrerntoheprecxpr-v tated as simpleinsoluble uorides. However, :omcanbetoleratedintheamonntsusual'lyfmmdmtap water, and comequently thiswater may be med for preparing the solutions. Using the solutiom withthe above groupofacidspresentinthestrengthstheconcenu'ationofthelluorideionmustbeatleast0.9%andcanbeedinquantitiesbelowandabovesaturationwhichisappmximatelyoftheorderofl0%thoride ion. 'IheprefcrredrangeofoonoentraonoitheuorideionhasbeenfoundtobebetweenaboutOi to8%.

Articlesrnaybetreatedeitherbysprayingorbyimmersion and in handling ofthe solutions rubber or polythc-lined containers are as the solunom arecorrosive.

Entirely satisfactory coatings can be formed without any activation orchemical of the surfaoett However, it is oftentimes tirs! to pickle the:nume with a solution hydro- A 5% hydmduorieacidsolntitmhasbeenfoundtobeamefulpickle. Insomecasesthe nature of the picklesolution may be a factor in obtaining the most successful coating and insuch cases its nature must be determined by the particular titaniumalloy to be processed. Any grease that may be present on the surface asreceived for coating will generally be removed by this picltling, but ifnecessary, a preliminary degreasing treatment may also be applied. lfthe titanium has been annealed it may carry a heavy scale, and this, ofcourse, should be removed also.

From the evidence that is available, the coating action of thesesolutions appears to be the result of the forma tion of a thin layer ofsolution saturated with titanium at the surface thereof under treatment.Accordingly, when a fresh solution is brought into contact with themetal surface there is a time lag before coating starts while thetitanium metal is being dissolved from the surface. It is thereforepreferred to add titanium to the solution. It has been found that thiscan be accomplished conveniently by adding to the solution just prior toimmersion of the metal enough uotitanate compound to saturate thesolution and simple tests under operating conditions can be made todetermine the amount required in a particular operation. A tluotitanatecompound such as an alkali lluotitanate, as for example, potassiumuotitanate can also be included in the coating solution initially and itcan be mixed with the solid compositions pre viously mentioned formaking up the solutions.

The solutions can be prepared by dissolving three in gredients in water,viz., one of the above group of acids,atuon'deandanalkalimetalcompoundandinthecase of hydrotuoric acid, twoingredients, viz., the acid and an alkali metal compound. Alternatively,in the case of the other acids, two ingredients can also be used, viz.,theacidandanalkalimetaluoride.

With respect to the above group of acids, while they an: substantiallyequivalent in their e'ect on the type ofcoatingformedeachorcertainoftheacidsofthegroup have specific advantageswhen used in the solution aloneandthispropertyofcertainoftheacidsmakesnatural subgroupiugs from theabove group of acids. The use of hydrouon'c acid without the others oithe group intbesolutionasthepredominantorsolesourceofthe acidity thereofhas the additional advantage that the complex uoride coating formed doesnot contain any other acidorsaltthereofenuainedorotherwisecarried overfrom the solution. The main advantage (in addition, ofcourse,tothosedescbedfortixegroupasawhole) in using alone the othermineral acids, namely, hydrochloric,niu'icandsulphmicacid,isthattheyarecheapand readilyavailable.'Iheadvantageoftheuseoftheorganicaddshavingadiociationconstantassetforthabove without the others as the sole source of theacidityofthesolntionisthattheeonn'olrequiredtomaintainthepHwithintherequiredrangeissmpler.

Hereinafter,thenvntionwillbedescn`bedwithrespccttoeachoftheabovembgroupsoftheacidsofthe generalgroup. lntbecaseofhydrotuorkacd, the solutionnbeprtparedbydidvinginwatertheaddandan alkali metal compound. Altcmativdy, a Solution of an alkali metalbnoride can be used to provide the es sentia! constituents of thissolution. In accordance with the feature of the invention wherein solidmaterials are employed to facilitate storage and transportation, a solidcomposition can be made by mixing a solid binonde and a solid compoundo( an alkali metal which whendissolv'ednwates-,ormanaqieoussolntiomwillprovidea woing solution. Ifthe cation of the solid biuoride isanalkalimetahanglesoiidcompotmdcanbemedfordzissolurionwhichmaktheprocessextremelysimple andconvenient.Forcxample,a 15% solution made frumwlidpotassinmbiuorideanda 12%solutionmade from solid sodium biuoride each produced a strongiy adhauztbrown-gray coating on titanium.

lnthemeoftheotherminaaladdshydrochloric,

nitric and sulphuric acid, the solution can be prepared from threeingredients, viz., one of these acids, a fluoride and an alkali metal.Alternatively, one of these acids and an alkali metal uoride only needbe used to supply the solution. Advantageously, a solid composition canbe used as the source of these acids and this feature will beillustrated with respect to sulphuric acid. In the case of sulphuricacid, a bisulphate or sulphamic acid can be used as the source ofsulphuric acid in the solution. As these are solids, their use hasadvantages and an example of such a composition comprises a. solidmixture of a solid bisulphate or sulphamic acid, a solid uoride and asolid alkali metal compound. This will provide a work;A ing solutionwhen dissolved in water or an aqueous solution. An even simpler solidmixture comprises two compounds, e.g., a bisulphate and an alkali metaluoride. Good results were obtained from a solution prepared from amixture which comprises 5 parts by weight of solid sodium bisulphate and4 parts by weight of solid sodium fluoride which is dissolved in waterin the quantity of 9 parts by weight of the mixture to make parts byweight of solution.

In the case of the organic acids which have a dissociation constantbetween lXl0- and 3Xl03, the solutionstobebroughtntoeontaetwiththemetalsnrfacecanbe prepared by dissolving inwater three ingredients, namely, the organic acid, a compound yieldingfluoride ions and an alkali metal compound. If desired, two ingredentscan be used, namely, the acid and an alkali metal tluoride. Ether asolution ready for use or a concentratewhichisdilutediustpriortousecanbeprepared. As some of the organic acids,e.g., citric and tartar-ic acids, are solids their use in such formprovides the advantages previously set forth for the solid materials. Asolid composition according to this feature of the invention comprises amixture of a solid organic acid having a primary dissociation constantbetween 1x10-4 and 3x10-z with a solidofone ormore ofthe compoundsproviding the fluoride ion and the alkali metal ion when dissolved. Evengreater simplification of theprocessarisesfromtheuseofasolidmixturewhichconsists of only twocompounds, as for example, citric acid and sodium or potassium uoride.

The following examples illustrate the improved acid fluoride coatingsolutions of the present invention and method of applying these solutiomto surfaces of titanium1 andaregivenwithrespecttoeachoftheabovesub`groups of acids, namely, hydroiluoric acid, and the ruineral acids,hydrochloric, nitric and sulphuric acid, and the organic acids.

Example I Coatings were produced on titanium by the various hydrotluoricacid or hydmiluoric acid-pr0ducing solutions set forth below by picklingtitanium panels for 30 seconds in a solution of nitric acid andhydrouoric acid, rinsingthemandthenimmcrsingthaninthevarrmssolutionsatroomtemperanneuntilgassingstopped. Thesolutionsandthecoatingweightsohtainedwere:

5 Example 2 The solution consisted of:

Percent H2904 1.5 'KF 5 NaNO, 0.25 XiTiP. 0.1

Water t make 100%.

Ahavinggoodadhesionanddarkgmyinap- 'pearanoewaspmdudinmimnesatZOCExample 3 The sohxtion eom'sted of:

Percent KHSO. 7 KF NaNO, 0.25 KJEF. 0.1

Watcrto make 100%.

Thesamepmducxwaansedindnahouampte .andtheaamensuhsweobinei Example4 Thesolutionof:

Watertomake im.

'lbsurfaofthewirerennnedmgoodoonditonrhlrtlgthedmwmgmmelnmusetfonhnhove.

Exampk Theaolunnof:

Pemem Gil'td 7 mik lo Wata'tomkelm.

Daxkadhexmwueornndontitaninmin Erample7 Theaolutiond:

Puvmt PlmnlicacL 5 Imi-ile l0 Wmertomakelm.

DarkafllereatwerefmedoninS ninnesltZOC.

Water to make DarkmyooatingsofgoodadhionwereformedontitaniuxninSminutesatZD C.

ltwllthusbeseenthatthcrehasbeenprovided by invention, an article, acomposition and axmethod in which the various objects hereinabove setforth, together with many thoroughly practical advantages aresuccessfullyachieved.

Sincatainchangesmaybemadeintheaboveinvention and different embodimentsof the invention may bemadewithoutdepartingfromthescopehcreotisintendedthatallmattercontainedintheabove-describeddisclosureshallbeintexpretedasilbtrativeandnotinaWhatisdaimedis:

l. Anaqueoacidicsolution for pmducingaooatingonsurfacoftitaniumandlitaniumbaaealloyshavingapHbetweenZandandgofatleastabontO oftheuoridion,atlstabont0.5%ofthei0nsofatleastoneoftheaikalimetalgandanacidselected''om the group of hydrouoxicacid, hydrodoxic acid,ntcacid,amisulphuxicacid,thcoonoenn-ationofsaidacidhnthesolutionbdngsuchthatthepanoftheacidityofthevsolutionisderivedtmmsaidadd.

LAnaqueousacidicsolutonforproducingawatingonsurfasoftitaniumandtitaniumbasealloyshavingapHbetween2and6andofatleastabout0.5% ofthelwrdematkastaboutoftheonsofatleastoneofthealkalimetals,andanacidselectedfmm the group of hydmuorieacid, hydrochloric adinitcadiandsulphncaciilcncmtrationofsaidaeidhtthesolutionbcingsurhthattbe.partoftheacidityofthesolmionisdctivedfmmsaid acid.

3. An for-producing aooatng onsurfasoftitaniumandbmealloyshavingapHbdWCellZvand 63nd HHC'HH; d3b0l0.9% @8%nodeiomat-leastabomlionsofatlestoneofofsaidadinthewhlonbdngsnchthattheprtpanzoitheacidtyofdtesohxtion'deved'ommid acid.

4. An laqueous add: solution for actuating base alloyshavngaonsurfaoesofttaniumandtitanium pHbetwem2and6andofabout0396to896mxxidematleastabout0596ioofatleastoneof bengsuch thatthepnepartoftheacidityofthesolutonisdevedfmm acid.

amd

$.Anaqueousacidicsolutionforpmducingaooatingonsnrfasofttanimnandtitanimnmealloyshavlgap}{betwen2and6andofabout0.9%to8%uorideiomatletaboutojkvofatleastoneofthe ofsaidaddinthesohltionbeingsuchthatthepanofdxeacidityofthesdutionsderivedfmmsaidadd.

6 AnaqneousaddicsolutionforaotingonsmiasoftitaninmandtitanumbasealloyshzvmgapHbetwoen2and6andofabout0.9%to8%horideiomatleastabontij'iionsofatleastoneofpartoftheacittyofthesoluonisdexivedfmmsaid and.

7.Asolidoompostionsolublemwamsoastoform asolutioncapableofanadhemonsurfasoftitanimnandhaseaoya oflanxuneofabinordganalkalimaalandainanamomnsutcanto saturate said solution, the proportions of saidbiuoride, alkali metal compound and uotitanate being such as to producean aqueous acidic solution having a pH between 2 and 6 and containing atleast 0.5% of the uoride ion andatleast0.5`% of theaikalimetalion.

8. A solid composition soluble in water so as to form a solution capableof producing an adherent coating on surfaces of titanium and titaniumbase alloys consistingofamixtureofatleastoneoompoundofthegoupsulphamicacidandabisulphatgaooridqandanalkalimetal the proportions of said acid, fluoride,andalkalimetalcompoundbeingsuchastoproduceanacidicsolutionhavingapHbetween2and6and containngatleast0.5%oftheiluorideionandatleast 0.5%ofthealkalimetalion.

9. Asolidcompositiousolubleinwatersoastoform a solution capable ofproducing an adherent coating onsnrfacesoftitaniumandtitaniumbasealloysoonsistingmeutiallyofamxtureofatleastoneoompoundofthegroupsnlpharnicacidandabisulphate,auoride,analkali metal compound, andauo'tanatein anamountsn cienttoaatnratesaidsolutiomthepxoportonsofsaidacid,ilnu|ide, andalkalimetaloompoundbeingmchastoproduanaqoeousacdicsolutonhavngaplibetwem2md6andcontaixngatleast0.$%oftheuoride ionandatleast05%ofxealkalimetalion.

10. AmethodfprodncinganadherentcoatingontbesurfacesoftitaniumandtitaniumbasealloyscomprisngsaidsmfastotheaclionofanaqueomacidicsolutionhavingapHbaween2and6andconsistingofatleastaboot0596oftheuoridciomatleastabont059oftheionsofatleastoneofdiealkalimetalgandan ad selected fromthegxonpofhydrouoric tlnofsaidacidinthesolutionbeingsnchthatthepartoftheacidityofthesolutionis lLAmethodormadha'entcoatii-igonleslxfasofandhnimnbealloyscompris'ngaaidmfaotntheaetionofanaqueoapHbetween 2 and 6 and ofabout09%t08% oftheun'ideioma:leastaboutooftheionsofatleastoneofthealkali metals, andlrydrohocmtheofsaid acidineadutionbdngsuchthatthepart oftbeadyoftlnisdiwdomsaidadd.

8 consistingoiabout0.9%to8% oftheuorideiomatoftheacidityofthesolutionisdervedomsaidadd. 13. Amethod forproducnganadherent coatingon the surfaces oftitaniumandtitaniumbasealloyscomousacidncsolutxonhavmgaplbetween2and6andcormstingofabout0.9%to8%ofthenondeion,at leastabout0.5%oftheiomofatletoneofthealkah metalgandnitricadmtheconntrationofsaidacidmmetakandanacidselectedfromthe group hydrcmicadihydrochloricmnitcaciandmd' :.lri tionbeingsndnthatthepartoftheacidity MecnsinthclcofthispatentUNITEDSTAIESPATEN'IS Panepinto Nov. 22, 1949 Amnndsen May l, 1951 Beachlune 2l, 1955 Miller et al. Doc. 16, 1958 O'I'HHIREFERBCESnalReportGlmSurfzTreatmemofTnaniuninWatawwnAnmalbyBattelleMemmiallmtimOctoherSO, 1953,30pages.

*lwonsacidics'dutinnhmingaplbetwemzandand 50 Amlunell,l95l,p.l52.

1. AN AQUEOUS ACIDIC SOLUTION FOR PRODUCING A COATING ON SURFACES OFTITANIUM AND TITANIUM BASE ALLOYS HAVING A PH BETWEEN 2 AND 6 ANDCONSISTING OF AT LEAST ABOUT 0.5% OF THE FLUORIDE ION, AT LEAST ABOUT0.5% OF THE IONS OF AT LEAST ONE OF THE ALKALI METALS, AND AN ACIDSELECTED FROM THE GROUP CONSISTING OF HYDROFLUORIC ACID, HYDROCHLORICACID, NITRIC ACID, AND SULPHURIC ACID, THE CONCENTRATION OF SAID ACID INTHE SOLUTION BEING SUCH THAT THE PREDOMINANT PART OF THE ACIDITY OF THESOLUTION IS DERIVED FROM SAID ACID.